In modern circuit-switched telephone networks, colloquially known as Plan Old Telephone Service (POTS), voice Codecs (Coder/Decoder) and Subscriber line Interface Circuits (SLIC) are used to interface with phone handsets, as well as analog and facsimile devices. Typically, the Codecs and SLICs reside in line cards in a central exchange (CO) or a Private Branch Exchange (PBX), driving handsets in customer premises at some distance away. Currently, an 8 kHz National Timing Reference (NTR) is used to provide a synchronized clock reference to all Codecs in all line cards in COs and PBXs for reliable telephonic operation.
Special equipment is used at the CO to gather the atomic-clock-generated NTR information from reliable sources, such as the National Timing Bureau, and to distribute it to all line cards. If no NTR signal is provided to the line cards, sampling clocks in the Codecs in the transmit and receive devices go out of synchronization, eventually losing significant data over some period of time. As a consequence, voice quality degrades and there is no reliable facsimile (fax) or analog phone modem transmission.
An illustration of a POTS system is shown in Prior Art FIG. 1A. Telephone set 101 communicates by analog voice to CO 102 which contains Codec 120. The NTR signal received at the CO is used by Codec 120 as a timing reference to decode analog voice (or FAX or video) data for digital transmission over long distances 200 to receiving CO 103 where the information is decoded by Codec 121 and synchronized by reference to the NTR data gathered at the receiving. CO. It is then sent to receiving telephone set 104.
Because most analog data streams, such as voice and video, are real-time and continuous, the information transmitted is normally generated by the source device and received by the destination device at a synchronized fixed rate. If the source and destination clocking is not synchronized, meaning the devices are not running in synch to the same reference clock, there will be a loss of information as one side overruns and the other side under-runs.
To ensure reliable communication of analog data over digital networks, a single synchronous master clock source needs to be provided to prevent data corruption and data loss when receiving and transmitting.
For Voice Over Internet Protocol (VoIP) applications, including voice, fax, video and analog modem, analog data are re-packetized into IP packets and transmitted over the internet. The Codec devices and SLIC circuits used in the VoIP equipment, unlike POTS, no longer reside in the COs and PBXs, but reside at the Internet Service Provider (ISP) sites, as well as user premises, instead.
An illustration of a typical VoIP implementation is gained by reference to Prior Art FIG. 1B. Headset 101 is connected through computer 105 by Local Area Network (LAN) 106 to server 107. Server 107 is connected by modem to CO 102 which in turn connects via POTS to ISP 201. ISP 201 is connected via internet 100 to ISP 202 which connects to CO 103. CO 103 provides, via POTS in this illustration, access to server 110 by LAN 111 to receiving user's computer 112 and finally telephone handset 104. Though in this illustration NTR is available at both COs, it isn't available in either LAN, resulting in possible “de-synchronization”. A more problematic situation exists when either or both LANs connect directly to the Internet with out employing an NTR-gathering CO. This is the situation illustrated in Prior art FIG. 1C, where LAN 106 and LAN 111 both connect directly to internet 100.
Many user sites are LAN-based, usually in some implementation of Ethernet. Ethernet does not normally carry a time reference signal. In fact, one advantage of the several Ethernet protocols is that they primarily support non-time-dependent communications, thus adapting to high and low density usage periods without apparent impact. Unfortunately, There is currently no convenient way to distribute NTR to VoIP Codecs in ISP and customer sites in Ethernet-connected networks without a large cost in bandwidth used. As a consequence, VoIP applications face big challenges in the distribution of NTR clock references to Codecs in customer premises equipment applications. Today, expensive fax relay and analog modem relays are common means to bypass this lack of the NTR provisions in VoIP applications.
Presently in the POTS environment, analog modem data or fax data are transmitted as analog data in the voice band, sometimes re-digitized for telephone system transmission and re-converted to analog at the receiving-end CO, and converted back to digital data at the receiving user's site. This scheme simplifies the telephone communication (Telco) equipment required, since there is no difference between actual voice data or analog and fax modem data as far as the phone network is concerned. However, it does result in conversion, coding, and decoding delays. As long as NTR is present in the network, though, reliable operation is guaranteed.
Fax and modem relays are schemes employed to overcome the potential loss of data in VoIP applications, primarily because of the lack of NTR provisions. In these schemes, fax or analog modem transmission is terminated at the VoIP equipment in the customer premise, relaying data received over the Internet to the customer's destined fax machine or analog modem. A significant amount of digital signal processing (DSP) is required both at the transmitting and receiving VoIP equipment for analog modem or fax relay implementation.
Any of the methods of providing VoIP without NTR are limited, and any method for providing NTR to network-based users is expensive, both in equipment costs and in Internet bandwidth usage. What is needed, therefore, is a method for providing a synchronization clock reference to Ethernet-based users that does not unnecessarily eat excessive bandwidth but does provide for high quality communication of time-dependent data.